Communication means are well known wherein infrared light or visible light is used and space serves as the signal transmission medium (see JP (Kokai) [Unexamined Japanese Patent Publication] 1[1989]-264,420). Such communication means have an advantage in that cables, fibers, and other types of wiring are not needed and this means can respond flexibly to changes in configuration, and the like. Communications in one or both directions is possible with a system whereby light emitted from a light-emitting side is received at a light-receiving side. A laser, a light-emitting diode, or another type of semiconductor device can be used as the light source.
On the other hand, studies are being performed on communications equipment that uses fluorescent lamps, light-emitting diodes, and other illumination apparatuses for indoor lighting (see, for example, JP (Kokai) [Unexamined Japanese Patent Publication] 1[1989]-292,918; JP (Kokai) [Unexamined Japanese Patent Publication] 11[1999]-163,795; JP (Kokai) [Unexamined Japanese Patent Publication] 2000-209,156; and JP (Kokai) [Unexamined Japanese Patent Publication] 2004-147,063). By means of this technology, it is possible to communicate with each terminal inside a room using a communication means in juxtaposition with an illumination apparatus.
Studies have been conducted in the field of light communication technology on reducing the effect of illumination light on communications using the periodicity of light emission, and the like. (See, for example, JP (Kokai) [Unexamined Japanese Patent Publication] 1[1989]-264,420; JP (Kokai) [Unexamined Japanese Patent Publication] 1[1989]-292,918; JP (Kokai) [Unexamined Japanese Patent Publication] 11[1999]-163,795) However, in the case of communications using visible light, even in places with relatively low illumination intensity, interference between the illumination light and communication light occurs, and high-reliability communication is not possible. For instance, communication using infrared light is possible in a location where the illumination intensity has temporarily decreased, but this is not possible in the case of visible light communications. Moreover, a further reduction in noise is needed in the case of infrared communication between terminals.
This problem becomes particularly important when two-way communication is necessary. Problems will rarely occur when lighting equipment is used for light communications and modulated light waves are emitted from lighting equipment in one direction (transmission of regional data or release of data specifying a location, or another application) as long as the lighting equipment continuously emits light. However, light power generated by the lighting equipment interferes with communications when there is a communication device at the same place and communication occurs between this device and the lighting equipment or the other communication device.
Furthermore, the illumination apparatus used as the light communication means must maintain both a communication function and an illumination function. That is, when this illumination apparatus is on, the apparatus must maintain optimal luminosity whether the communication function is on or off. The illumination apparatus usually receives electrical power from an alternating-current power source.
Therefore, an object of the present invention is to realize with a relatively a simply configured light communication system and an illumination apparatus therefor, with which high-reliability information exchange is possible by light communication between an illumination apparatus and a terminal, or light intercommunication between terminals.